The displacement sensor for measuring a displacement such as a dimension of an object according to a light section image (image obtained by using a light section method) is known in the art. Typically, a displacement sensor of this type automatically extracts a coordinate of a measuring point from an image obtained by using an imaging device in a sensor head according to a prescribed measuring point extraction algorithm, and computes a desired displacement from the automatically extracted measuring point coordinate.
The sensor head is incorporated with a laser diode for emitting a spot beam (beam having an extremely small circular cross section) or a line beam (beam having a linear cross section), and an imaging device (one-dimensional CCD, two-dimensional CCD or the like) for monitoring a region containing a radiation point of the beam from a different angle, and producing an image containing a variation corresponding to a displacement of the monitored object.
The main unit automatically extracts a measuring point coordinate from the image obtained from each sensor head according to a measuring point extraction algorithm designated by the user. Then, the actual displacement is computed from the automatically extracted measuring point coordinate by using a triangular computation or the like. If a range of variation tolerance (threshold value) is defined for the displacement, a tolerance determination process is performed, and a binary signal indicating the acceptability of the object can be obtained.
According to such a conventional displacement sensor, if the measuring point extraction algorithm or the like is designated in advance, a measuring point coordinate can be automatically extracted from the image obtained, and the desired displacement can be computed in the end so that no effort is required for the user.
However, it does not provided any means for verifying the data (such as the raw image from the imaging device, the automatically extracted measuring point coordinate and various automatically defined threshold values or the like) which is used during the entire process of obtaining the image and computing the displacement.
Therefore, when the measured displacement turns out to be abnormal or when an unacceptable measurement result is obtained, it is not possible to distinguish for the user if it is due to the abnormal condition of the object or due to the abnormal operation of the sensor, possibly due to the influences of external light or the like.
Furthermore, the conventional displacement sensor is not capable of defining the field of view of the imaging device at will, for example, to extract a measuring point at will. This is a particularly significant problem of a displacement sensor with two-dimensional imaging devices (such as two-dimensional CCD or the like). Specifically, because, in normal cases, there are a plurality (typically in the order of tens) of columns of pixels extending in the direction of the displacement in a mutually parallel relationship, the need for finding a peak point and a bottom point from each of these columns creates a serious difficulty when implementing the sensor as a commercial product.